ISOPHOT, the imaging spectro-photo-polarimeter on board ISO, supported performance of aperture and mapping photometry, low-resolution spectroscopy and polarimetry, both in staring and chopped mode. In addition, the 170 μm ISOPHOT Serendipity Survey was performed while slewing from one target to the next. Its calibration had to be connected to the pointed and raster observations. One initial aspect of the calibration program was the finding of optimal observation strategies (chopping, mini-maps, absolute photometry) depending on source type. FIR detection limits were determined facing the in-orbit radiation environment and the inhomogeneous sky background. ISOPHOT's large wavelength range from 2.5 to 240 μm, covered by 25 filters in combination with subsets of 17 different apertures or array/pixel sizes, combined with a large dynamic range set the frame of requirements on calibration measurements and standards. Based on the pre-flight characterisation a list of requirements was worked out as a guide for the calibration planning. An important item in the absolute photometric calibration was the reference to celestial standards via a transfer calibration using the internal reference sources. This became particularly demanding when one of the internal reference sources changed its radiation properties in the early routine phase. We give an overview of standard sources, calibration strategies for each main observational mode and the accuracies achieved so far. With the end of the ISO Post Operations Phase the scientific validation status of most of the ISOPHOT observing modes has been established.